1. Field of the Invention
Generally, the present invention relates to ultrasound imaging. More specifically, the present invention relates to microfabricated transducers with associated electronics capable of three-dimensional ultrasound imaging.
2. Description of the Related Art
An acoustic transducer is an electronic device used to emit and receive sound waves. Ultrasonic transducers are acoustic transducers that operate at frequencies above 20 KHz, and more typically, in the 1-20 MHz range. Ultrasonic transducers are used in medical imaging, non-destructive evaluation and other applications. The most common forms of ultrasonic transducers are piezoelectric transducers. In U.S. Pat. No. 6,271,620 entitled, “Acoustic Transducer and Method of Making the Same,” issued Aug. 7, 2001, Ladabaum describes microfabricated ultrasonic transducers (MUTs) capable of competitive performance compared to piezoelectric transducers.
In U.S. Pat. No. 6,246,158, Ladabaum teaches monolithic integration of MUTs with circuitry. The basic transduction element of the MUT is a vibrating capacitor. A substrate contains a lower electrode, a thin diaphragm is suspended over the substrate and a metallization layer serves as an upper electrode. If a DC bias is applied across the lower and upper electrodes, an acoustic wave impinging on the diaphragm will set it in motion, and the variation of electrode separation caused by such motion results in an electrical signal. Conversely, if an AC signal is applied across the biased electrodes, the AC forcing function will set the diaphragm in motion, and this motion emits an acoustic wave in the medium of interest.
In U.S. Pat. No. 6,430,109. Khuri-Yakub et al. teach the use of through-wafer vias to provide electrical connections to MUT elements and thus allow connection to an image processing chip. The image processing chip is not described, and the through-wafer interconnects are taught to provide a means for control voltages and electrical excitation of MUTs. The focus of this referenced prior art is transmission; reception details are not taught.
Integration of piezoelectric materials with electronics is also known in the art, as is taught in Plummer, J., Meindl, J., and Maginness, M., “An Ultrasonic Imaging System for Realtime Cardiac Imaging,” Proceedings of the IEEE International Solid-State Circuits Conference, 1974, p. 162-163. PVDF (polyvinyl di-fluoride), a piezoelectric polymer, can be formed on an electronic substrate. See, for example, Reston, R. and Kolesar, E, “Pressure-sensitive field-effect transistor sensor array fabricated from a piezoelectric polyvinylidene fluoride film, Proceedings of the IEEE Engineering in Medicine and Biology Society, 1989, p. 918-919. The authors are not aware of high quality medical imaging piezoelectric materials successfully integrated directly on top of electronic circuits, such as PZT-5H, for example, though it may be that in the future high quality composite piezoelectrics are successfully formed on top of electronics. The present invention is taught with respect to a monolithically integrated MUT transducer embodiment, but need not be limited to it. Piezoelectric, through-wafer via MUTs, or other currently unknown transducer layers may be used on the electronic circuitry herein disclosed.
In U.S. Pat. No. 6,106,472, Chiang and Broadstone teach a system and method of beam formation within a probe housing. The beam formation is accomplished by the sampling, delay, and summation of ultrasonic channel data. The compact nature of the beamformer is made possible by a CCD delay chip.
None of these references teaches or claims specific structures or methods directed to 3-D imaging.
Savord et al., in U.S. Pat. No. 6,381,197, describe the use of both bias lines and FET switches to control the aperture of a MUT array, but do not teach specific structures or methods directed to 3-D imaging.
Ultrasound systems that generate three-dimensional (3-D) images of the subject of interest are available today. Most of the commercially available systems form three-dimensional images form multiple two-dimensional (2-D) slices taken by a mechanically translating or rotating probe. An example of such a system is General Electric's Voluson 730, which has its origins from the work of Kretz in Austria. U.S. Pat. No. 4,341,120, issued in 1982, describes a multi-element probe that is electronically scanned in the azimuth direction, but is mechanically moved to capture image slices in the elevation direction.
Mechanical translation suffers from several disadvantages, among them cost, reliability, and mechanical jitter. The resolution of the reconstructed image in the elevation direction is a function of the slice thickness of the elevation profile of the transducer, as well as of the positioning accuracy of the mechanical translation scheme or device.
Other approaches to 3-D imaging are also known in the art. Systems based on two dimensional transducer arrays are taught, for example, in U.S. Pat. Nos. 4,694,434, 5,229,933 and 6,126,602. One disadvantage of these conventional 3-D imaging systems based on 2-D transducer arrays is that the interconnecting circuitry between each individual transducer element and its associated control circuitry can be difficult and expensive to design and manufacture. Furthermore, parasitic resistance, capacitance, and cross-talk in the interconnect paths can degrade the performance of the imaging system.
In both mechanically scanned and electronically scanned approaches for 3-D imaging known in the art, a disadvantage is the slow frame-rate of 3-D images that is a function of a 3-D image being formed from many 2-D slices.
In imaging applications, an ultrasonic transmitter sends ultrasound waves into the subject of interest, and an ultrasonic receiver detects the return waveforms. Typically in medical imaging, the transmitter and receiver are the same transducer array, and timing between the elements of the array is varied during transmit and receive events to form images. However, the transmitter need not necessarily be the same as the receiver. It has been realized by the present inventors that as long as sufficient signal-to-noise ratio is available, a fully populated matrix of transducers can capture enough information during receive events to form 3-D images of the subject of interest. The present invention relates to the electronic circuitry used to control a two-dimensional matrix of transducers during receive events. The transducer matrix and electronics can be MUT transducers monolithically integrated with underlying electronics, as taught by Ladabaum, MUT transducers connected by through-wafer vias to the electronics, as taught by Khuri-Yakub, PVDF transducers formed over the electronics and connected to them, or other piezoelectric transducers not yet reduced to practice. It is critical that the interconnect paths between transducer element and electronics have insignificant parasitic resistance and capacitance. Thus, the monolithic approach is the preferred embodiment.
Thus, what is needed is electronic circuitry that can be used with a fully populated two-dimensional transducer array (matrix), preferably, for example, integrated immediately below the transducer matrix, and that can provide sufficient information for the formation of three-dimensional images by a 3-D ultrasound imaging system. The present invention provides such circuitry.